1. Field of the Invention
The present invention relates to systems for feeding a length of paper, and more particularly to systems for feeding a length of print paper through a print station within an impact printer.
2. History of the Prior Art
Impact printers typically include a system for feeding a length of print paper through a print station where impact printing on the paper takes place. The paper feed system may be of the type described in U.S. Pat. No. 3,941,051 of Barrus et al., which patent issued Mar. 2, 1976 and is commonly assigned with the present application. In the printer described in the Barrus et al. patent, the length of print paper is advanced upwardly from a paper ironer through a print station by a pair of tractor drives mounted above the print station. The tractor drives have pins on endless belts which engage spaced apart apertures at the opposite edges of the length of paper. A reciprocating hammerbank at the print station impacts the length of paper against a supporting platen through a length of ink ribbon to print on the paper in dot matrix fashion. The elongated paper ironer drags against the paper to maintain the paper in tension through the print station as the paper is advanced upwardly in incremental fashion by the pair of tractor drives. The direction of paper feed is not reversible.
The paper ironer of the printer described in U.S. Pat. No. 3,941,051 of Barrus et al. which maintains the paper in tension as it is fed through the print station can be replaced with other paper tensioning arrangements. Examples of more sophisticated paper tensioning arrangements are described in U.S. Pat. No. 4,687,359 of Barrus et al. which issued Aug. 18, 1987, and which is commonly assigned with the present application, and copending application Ser. No. 628,418 of Barrus et al. now abandoned, filed Jul. 6, 1984 and commonly assigned with the present application. As in the case of the paper ironer of the '051 patent of Barrus et al., the paper tensioning arrangements described in the Barrus et al. '359 patent and the copending application do not provide for bidirectional feeding of tile paper through the print station.
An example of a motor driven arrangement disposed on the opposite side of the print station from a pair of tractor drives and which may be employed to feed the paper bidirectionally through the print station is described in Japanese Patent No. 57-39980 of Sugano, dated Mar. 5, 1982. In the Sugano patent the motor driven arrangement operates bidirectionally at speeds slightly different from the speed of the tractor drives so as to maintain the paper within the print station in tension as it is fed in opposite directions.
A frequently used paper feed system for maintaining reasonably effective control of a length of print paper while feeding the length of print paper through a print station bidirectionally employs pairs of tractor drives of similar design above and below the print station. Typically, such tractor drives utilize belts, chains or other endless members having spaced apart pins for engaging the holes at the opposite edges of the length of paper. By locating different pairs of the tractor drives above and below the print station, the paper may be fed bidirectionally through the print station with a desired amount of tension and in a desired manner. However, in addition to being rather costly and requiring considerable space, such arrangements may still leave much to be desired in terms of the lack of precision with which they feed the paper. For example, in the case of line printers employing a reciprocating hammerbank within the print station, the hammerbank typically imposes substantial lateral loads on the paper making precise handling of the paper difficult.
In an effort to exercise closer and more accurate control over the bidirectional feeding of paper through a print station, paper feed systems have been devised in which each one of a pair of tractor drives extends on both sides of the print station and engages opposite portions of the length of paper at locations relatively close to the print station. Examples of such paper feed systems are provided by U.S. Pat. No. 3,392,893 of Bennett et al., which patent issued Jul. 16, 1968, and U.S. Pat. No. 3,908,883 of Bellisai et al., which patent issued Sep. 30, 1975. The Bennett et al. patent describes an arrangement which uses pulleys to position portions of an endless belt having pins spaced along the length thereof both above and below a print station. Phasing or pitch adjustment between the two portions is accomplished by a central pulley movable along a horizontal slot between the two different portions and a second pulley movable within a vertical slot and engaging the inside of the endless belt at a different location. The arrangement described in Bellisai et al. employs an endless chain having spaced apart pins along the length thereof and including a chain tensioning device. A central guide section disposes a central portion of the chain out of the way of the print station. A further example of an arrangement for adjusting phasing between opposite tractor drives is provided by U.S. Pat. No. 3,827,616 of Atkinson, which patent issued Aug. 6, 1974.
The patents of Bennett et al. and Bellisai et al. represent a step in the direction of more effective and efficient control of bidirectional paper feed through a print station. However, while the paper feed systems described in the patents dispose portions of integral tractor drives above and below and adjacent tile print station, such systems suffer from a number of disadvantages which limit their usefulness and effectiveness. One major drawback in the case of the Bennett et al. system, for example, is the inability of the system described therein to adjust the phasing between the portions on opposite sides of the print station after the paper is installed. Because the phasing must be adjusted using two different pulleys movable along mutually orthogonal slots, the phasing can only be adjusted prior to installation of the paper. Still other shortcomings of tile systems described in tile Bennett et al. and Bellisai et al. patents relate to the configuration, mounting and method of operation of such systems.
Various attempts have been made to provide improved paper feed systems through the introduction of various potentially advantages features. However, such features have usually fallen short in terms of solving the particular problems to which they are directed, and especially in terms of providing a unique overall combination constituting a truly improved paper feed system. Thus, in U.S. Pat. No. 4,344,715 of Van Horne et al. a tractor drive is mounted on one side of the platen and is locked in place by an arrangement of limited effectiveness. In U.S. Pat. No. 4,214,691 of van Namen, which patent issued Jul. 29, 1980, the pitch of the belt teeth is selected relative to the pitch circle of the pins on the belt in an effort to remove the pins from the apertures in the paper without tearing the paper. Also, U.S. Pat. No. 3,608,801 of Nystrand, which patent issued Sep. 28,1971, and U.S. Pat. No. 3,688,959 of Staneck et al., which patent issued Sep. 5, 1972, describe the provision of angled ramp portions at the ends of the tractor drive in an effort to facilitate pin removal from the paper. However, such arrangements concentrate on pin removal at the expense of the paper feed function itself, particularly at the ends of the tractor drive where some paper drive is still important. The Nystrand patent also attempts to maintain the paper in a close mating engagement with the moving belt and the pins thereon, but does so at the expense of requiring a completely separate belt and pulley arrangement.
Accordingly, it is an object of the present invention to provide an improved paper feed system.
A further object of the present invention is the provision of a paper feed system which provides for accurate paper positioning and movement in conjunction with bidirectional paper feed operation.
Further objects in accordance with the present invention include the provision of a paper feed system of versatile bidirectional design and which has first and last line and demand document capabilities.